Australian officials supervising the search for the missing Malaysia Airlines flight have said that an underwater search for the black box recorder based on "pings" possibly from the device could be completed in five to seven days.

It comes as search teams say the submarine currently scanning the ocean floor remains "the best lead" in finding the plane.

The US Navy-operated Bluefin-21 autonomous underwater vehicle (AUV) has now covered an area of 82 square miles (133 sq km) and has completed six missions.

But its sonar scanners have detected nothing, Sky News understands.

The Bluefin-21 submersible has already carried out six descents

After nearly six weeks without any sign of the plane, the current underwater search has been narrowed to a circular 6.2-mile (10km) area around the location where one of the pings thought to come from the missing flight's black box was detected earlier this month.

The submersible is likely to take up to a week to cover the refined search area.

The Bluefin-21 has now started its seventh descent to the bottom of the Indian Ocean.

It takes two hours for the unmanned submersible to travel more than 4,500m to the seabed where it spends 16 hours at a time using sonar scanners to map the ocean floor. Data is then downloaded at the surface.

The Chinese ship Hai Xin 01 conducts a search in the southern Indian Ocean

No sign of the plane has been picked up, said search officials based in Perth, Australia.

"Overnight, Bluefin-21 AUV completed mission six in the underwater search area. Data from the sixth mission is currently under analysis. No contacts of interest have been found to date," said an official.

"This is the best lead we have in the search for missing flight MH370."

Hishammuddin Hussein, acting Malaysia Transport Minister, told a news conference on Saturday: "The immediate search area that the Bluefin-21 is scouring should be completed within the next week.

"All efforts will be intensified in the next few days in regards to the search."

The underwater hunt is complicated by the depth of the largely unexplored sea floor. The US Navy's unmanned sub has already gone beyond its recommended limit of 4,500 meters (15,000 feet).

Malaysia flight MH370 took off from Kuala Lumpur on March 8 bound for Beijing. But an hour into the flight it disappeared from radar. There were 239 people on board, mostly Chinese citizens.

Some families of those on board refuse to believe the aircraft crashed into the sea and have instead denounced the search effort as a cover-up.

Up to 11 military aircraft and 12 ships are assisting the search over the long Easter bank holiday weekend. The total search area is 31,000 square miles (50,200 sq km), across three areas.